Rate responsive pacemaker and pacing method

Abstract

A rate responsive pacemaker and a pacing method for optimizing the pacing decay curve after a period of increased activity. The pacing method includes the steps of selecting a set of predetermined achievement criteria such as an achievement rate and an achievement time interval. The achievement rate is selected between an upper pacing rate and a first pacing switch rate threshold. The pacing method then determines whether the achievement criterion has been met. If the achievement criterion has been met, then the decay time constant of the decay curve changes from a first value to a second value, as the pacing rate drops below the first pacing switch rate threshold. A second pacing switch rate threshold lower than the first pacing switch rate threshold is then selected, and, if the achievement criteria have been met, then the decay time constant of the decay curve is modified from the second value to a third value, as the pacing rate drops below the second pacing switch rate threshold.

What is claimed is: 
     
       1. Method for pacing along a decay curve having a decay time constant, the pacing method comprising the steps of: a. selecting a set of predetermined achievement criteria;   b. selecting a first pacing switch rate threshold;   c. determining whether said achievement criteria have been met; and   d. if said achievement criteria have been met, then modifying the decay time constant of the decay curve from a first value to a second value, as the pacing rate drops below said first pacing switch rate threshold, such that said second value is different from said first value.   
     
     
       2. The method as defined in claim 1, further including the steps of: a. selecting a second pacing switch rate threshold lower than said first pacing switch rate threshold; and   if said achievement criteria have been met, then modifying the decay time constant of the decay curve from the second value to a third value, as the pacing rate drops below said second pacing switch rate threshold, such that said second value is different from said third value.   
     
     
       3. The method as defined in claim 2, wherein said second value of the time constant for the decay curve is longer than said first value, in order to allow a slower decay of the pacing rate. 
     
     
       4. The method as defined in claim 3, wherein said third value of the 5 time constant for the decay curve is substantially equal to said first value. 
     
     
       5. The method as defined in claim 1, further including the step of keeping the time constant of the decay curve unchanged if said achievement criteria have not been met. 
     
     
       6. The method as defined in claim 1, wherein said achievement criteria includes an achievement rate and an achievement time interval; wherein said achievement rate is a programmable percentage of the difference between a Lower Rate (LR) and an Upper Rate (UR); wherein said Lower Rate and Upper Rate are the lower and upper pacing rates respectively; and wherein the achievement time interval is a minimum time interval over which the pacing rate exceeds said achievement rate. 
     
     
       7. The method as defined in claim 6 wherein said achievement time interval is about 4 seconds. 
     
     
       8. The method as defined in claim 6, wherein said achievement rate is calculated as follows:   Achievement Rate=Lower Rate+a (Upper Rate-Lower Rate),     where "a" is a percentile value which ranges between 50% and 100%, and the Lower Rate and the Upper Rate are the lower and upper pacing rates respectively.   
     
     
       9. The method as defined in claim 6, wherein said achievement rate is 125 pulses per minute. 
     
     
       10. The method as defined in claim 2, wherein said first pacing switch rate threshold is calculated as follows:   First Pacing Switch Rate=Lower Rate+u (Upper Rate-Lower Rate)     where "u" is a percentile value which ranges between 20% and 50%, and where the Lower Rate and the Upper Rate are the lower and upper pacing rates respectively.   
     
     
       11. The method as defined in claim 10, wherein said first pacing switch rate is about 90 pulses per minute. 
     
     
       12. The method as defined in claim 10, wherein said second pacing switch rate threshold is calculated as follows:   Second Pacing Switch Rate=Lower Rate+10% Lower Rate.     
     
     
       13. The method as defined in claim 12, wherein said second pacing switch rate is about 77 pulses per minute. 
     
     
       14. The method as defined in claim 1, further including the steps of: a. using an activity sensor to sense cardiac events, and to generate an output indicative of the amplitude of the sensed cardiac events; and   b. calculating a target rate, wherein said step of calculating said target rate, is performed according to the following equation: ##EQU3## where C and D are selectively programmable values, and where said Activity Count is a measure of said activity sensor output over a predetermined interval of time.   
     
     
       15. A cardiac pacemaker of the type having a pulse generator for generating stimulus pulses, comprising: means for pacing along a decay curve having a decay time constant, comprising: i) means for selecting a set of predetermined achievement criterion;   ii) means for selecting a first pacing switch rate threshold;   iii) means for determining whether said achievement criterion has been met; and   iv) means for modifying the decay time constant of the decay curve from a first value to a second value, as the pacing rate drops below said first pacing switch rate threshold, such that said second value is different from said first value, if said achievement criterion has been met.   
     
     
       16. The cardiac pacemaker as defined in claim 15, wherein said pacing means further includes: a. means for selecting a second pacing switch rate threshold lower than said first pacing switch rate threshold; and   b. means for modifying the decay time constant of the decay curve from the second value to a third value, if said achievement criteria have been met, as the pacing rate drops below said second pacing switch rate threshold, such that said second value is different from said third value.